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Jocelyn Mailhot

Abstract

A mesoscale numerical simulation (35 km) of a return-flow event over the Gulf of Mexico that occurred during the Gulf of Mexico Experiment (GUFMEX) is presented in order to examine the structure and the transformation of the polar air mass and to assess the model's skill in simulating the event. The study deals with the phase of cold-air outbreak over the Gulf of Mexico and the subsequent rapid modification of the cold air mass by the underlying warm ocean, prior to the onset or return flow.

The investigation focuses on the physical processes operating during the airmass transformation, notably the air-sea fluxes and the vertical destabilization of the airman. The results are compared with various data gathered during GUFMEX and suggest that a realistic simulation of airmass transformation can be obtained. The results indicate a strong interplay between 1) large-scale subsidence above the planetary boundary layer behind the front and 2) destabilization near the sea surface and in the boundary layer. In particular, advective processes play a central role in the airmass modification above the boundary layer and in the maintenance of a strong capping inversion. However, very large surface energy fluxes and vigorous turbulent vertical mixing appear as dominant mechanisms within the boundary layer itself. A sensitivity experiment where surface energy fluxes are turned off supports these conclusions and clearly demonstrates their impact on the advance of the cold air mass over the Gulf and on the changes in moisture and stability of the return flow.

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Stéphane Bélair
and
Jocelyn Mailhot

Abstract

The relative roles of implicit and explicit condensation schemes in the numerical representation of a squall line that occurred on 7–8 May 1995 over the southern Great Plains are examined in this study using Mesoscale Compressible Community model integrations at 2-, 6-, 18-, and 50-km resolution. Results from the 2-km model in which condensation is explicitly represented agree best with observations and are used as “synthetic” data to evaluate the performance of lower-resolution configurations.

It is found that the representation of the squall system greatly deteriorates as resolution is decreased and that the relative roles of the implicit and explicit condensation schemes change dramatically. At 6-km resolution, the leading convective band is barely resolved by the model, and the implicit–explicit partition of precipitation is ambiguous because both implicit and explicit schemes are active simultaneously at the leading edge of the system. In spite of this ambiguity, it is found that use of a deep convection scheme is still beneficial to the squall-line simulation. At 18 km, the convective line is not resolved by the model, and its effect is completely due to the implicit scheme. The mesoscale circulations in the trailing anvil region of the squall system are generated at the small end of the model resolvable scales and are exaggeratedly intense. There is no ambiguity concerning the partition of condensation into implicit and explicit components at this resolution, but the relative intensity of precipitation produced by the two cloud schemes is opposite to what is observed, considering that the implicit scheme is supposed to represent subgrid-scale convection at the leading edge of the system, and the explicit scheme the grid-scale condensation in the trailing anvil. At 50 km, both the leading convection and the mesoscale circulations in the trailing anvil have to be parameterized because they are not resolved at the model grid scale. The precipitation and internal structures associated with the squall line are thus not well represented at this resolution.

The results also show that all the configurations produce precipitation accumulations that are much larger than observations. This problem is most important at 18-km resolution. Grid-scale condensation is mostly responsible for this rainfall overestimation. It is suggested that this problem is linked to a misrepresentation of convective-scale processes.

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Stéphane Bélair
,
Jocelyn Mailhot
,
Claude Girard
, and
Paul Vaillancourt

Abstract

The role and impact that boundary layer and shallow cumulus clouds have on the medium-range forecast of a large-scale weather system is discussed in this study. A mesoscale version of the Global Environmental Multiscale (GEM) atmospheric model is used to produce a 5-day numerical forecast of a midlatitude large-scale weather system that occurred over the Pacific Ocean during February 2003. In this version of GEM, four different schemes are used to represent (i) boundary layer clouds (including stratus, stratocumulus, and small cumulus clouds), (ii) shallow cumulus clouds (overshooting cumulus), (iii) deep convection, and (iv) nonconvective clouds. Two of these schemes, that is, the so-called MoisTKE and Kuo Transient schemes for boundary layer and overshooting cumulus clouds, respectively, have been recently introduced in GEM and are described in more detail.

The results show that GEM, with this new cloud package, is able to represent the wide variety of clouds observed in association with the large-scale weather system. In particular, it is found that the Kuo Transient scheme is mostly responsible for the shallow/intermediate cumulus clouds in the rear portion of the large-scale system, whereas MoisTKE produces the low-level stratocumulus clouds ahead of the system. Several diagnostics for the rear portion of the system reveal that the role of MoisTKE is mainly to increase the vertical transport (diffusion) associated with the boundary layer clouds, while Kuo Transient is acting in a manner more consistent with convective stabilization. As a consequence, MoisTKE is not able to remove the low-level shallow cloud layer that is incorrectly produced by the GEM nonconvective condensation scheme. Kuo Transient, in contrast, led to a significant reduction of these nonconvective clouds, in better agreement with satellite observations. This improved representation of stratocumulus and cumulus clouds does not have a large impact on the overall sea level pressure patterns of the large-scale weather system. Precipitation in the rear portion of the system, however, is found to be smoother when MoisTKE is used, and significantly less when the Kuo Transient scheme is switched on.

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Yi-Ching Chung
,
Stéphane Bélair
, and
Jocelyn Mailhot

Abstract

A one-dimensional (1D) version of a blowing snow model, called PIEKTUK-D, has been incorporated into a snow–sea ice coupled system. Blowing snow results in sublimation of approximately 12 mm of snow water equivalent (SWE), which is equal to approximately 6% of the annual precipitation over 324 days from 1997 to 1998. This effect leads to an average decrease of 9 cm in snow depth for an 11-month simulation of the Surface Heat Budget of the Arctic Ocean (SHEBA) dataset (from 31 October 1997 to 1 October 1998). Inclusion of blowing snow has a significant impact on snow evolution between February and June, during which it is responsible for a decrease in snow depth error by about 30%. Between November and January, however, other factors such as regional surface topography or horizontal wind transport may have had a greater influence on the evolution of the snowpack and sea ice. During these few months the new system does not perform as well, with a snow depth percentage error of 39%—much larger than the 12% error found between February and June. The results also indicate a slight increase of 4 cm on average for ice thickness, and a decrease of 0.4 K for the temperature at the snow–ice interface. One of the main effects of blowing snow is to shorten the duration of snow cover above sea ice by approximately 4 days and to lead to earlier ice melt by approximately 6 days. Blowing snow also has a very small impact on internal characteristics of the snowpack, such as grain size and density, leading to a weaker snowpack.

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Yi-Ching Chung
,
Stéphane Bélair
, and
Jocelyn Mailhot

Abstract

The new Recherche Prévision Numérique (NEW-RPN) model, a coupled system including a multilayer snow thermal model (SNTHERM) and the sea ice model currently used in the Meteorological Service of Canada (MSC) operational forecasting system, was evaluated in a one-dimensional mode using meteorological observations from the Surface Heat Budget of the Arctic Ocean (SHEBA)’s Pittsburgh site in the Arctic Ocean collected during 1997/98. Two parameters simulated by NEW-RPN (i.e., snow depth and ice thickness) are compared with SHEBA’s observations and with simulations from RPN, MSC’s current coupled system (the same sea ice model and a single-layer snow model). Results show that NEW-RPN exhibits better agreement for the timing of snow depletion and for ice thickness. The profiles of snow thermal conductivity in NEW-RPN show considerable variability across the snow layers, but the mean value (0.39 W m−1 K−1) is within the range of reported observations for SHEBA. This value is larger than 0.31 W m−1 K−1, which is commonly used in single-layer snow models. Of particular interest in NEW-RPN’s simulation is the strong temperature stratification of the snowpack, which indicates that a multilayer snow model is needed in the SHEBA scenario. A sensitivity analysis indicates that snow compaction is also a crucial process for a realistic representation of the snowpack within the snow/sea ice system. NEW-RPN’s overestimation of snow depth may be related to other processes not included in the study, such as small-scale horizontal variability of snow depth and blowing snow processes.

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Stéphane Bélair
,
Da-Lin Zhang
, and
Jocelyn Mailhot

Abstract

In an effort to improve operational forecasts of mesoscale convective systems (MCSs), a mesoscale version of the operational Canadian Regional Finite-Element (RFE) Model with a grid size of 25 km is used to predict an intense MCS that occurred during 10–11 June 1985. The mesoscale version of the RFE model contains the Fritsch–Chappell scheme for the treatment of subgrid-scale convective processes and an explicit scheme for the treatment of grid-scale cloud water (ice) and rainwater (snow).

With higher resolution and improved condensation physics, the RFE model reproduces many detailed structures of the MCS, as compared with all available observations. In particular, the model predicts well the timing and location of the leading convective line followed by stratiform precipitation; the distribution of surface temperature and pressure perturbations (e.g., cold outflow boundaries, mesolows, mesohighs, and wake lows); and the circulation of front-to-rear flows at both upper and lower levels separated by a rear-to-front flow at midlevels.

Several sensitivity experiments are performed to examine the effects of varying initial conditions and model physics on the prediction of the squall system. It is found that both the moist convective adjustment and the Kuo schemes can reproduce the line structure of convective precipitation. However, these two schemes are unable to reproduce the internal flow structure of the squall system and the pertinent surface pressure and thermal perturbations. It is emphasized that as the grid resolution increases, reasonable treatments of both parameterized and grid-scale condensation processes are essential in obtaining realistic predictions of MCSs and associated quantitative precipitation.

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Stéphane Bélair
,
Louis-Philippe Crevier
,
Jocelyn Mailhot
,
Bernard Bilodeau
, and
Yves Delage

Abstract

The summertime improvement resulting from the operational implementation of a new surface modeling and assimilation strategy into the Canadian regional weather forecasting system is described in this study. The surface processes over land are represented in this system using the Interactions between Soil–Biosphere–Atmosphere (ISBA) land surface scheme. Surface variables, including soil moisture, are initialized using a sequential assimilation technique in which model errors of low-level air temperature and relative humidity are used to determine analysis increments of surface variables.

It was found that the magnitude and nature of the analysis increments applied to the surface variables depended on the surface and meteorological conditions observed in each region. In regions characterized by weak meteorological activity (i.e., no clouds or precipitation), model errors of low-level air characteristics are more likely to be related to an incorrect representation of surface processes due to either erroneous initial conditions or inaccurate parameterizations in the land surface scheme. In other regions characterized by more frequent and more intense precipitation events, surface corrections are mainly associated with inaccurate atmospheric forcing.

Objective evaluation against observations from radiosondes and surface stations showed that the amplitude of the diurnal cycle of near-surface air temperature and humidity is larger with the new surface system, in better agreement with observations. This type of improvement was found to extend higher up in the boundary layer (up to 700 hPa) where cold and humid biases were significantly reduced by introducing the new surface system. The model precipitation was also found to be significantly influenced by the new representation of surface fluxes. The problematic increase of a positive bias in precipitation with integration time was found to be significantly reduced with the new system, due to the warmer and drier boundary layer.

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Stéphane Bélair
,
Ross Brown
,
Jocelyn Mailhot
,
Bernard Bilodeau
, and
Louis-Philippe Crevier

Abstract

The performance of a modified version of the snow scheme included in the Interactions between Surface–Biosphere–Atmosphere (ISBA) land surface scheme, which was operationally implemented into the regional weather forecast system at the Canadian Meteorological Centre, is examined in this study. Stand-alone verification tests conducted prior to the operational implementation showed that ISBA's new snow package was able to realistically reproduce the main characteristics of a snow cover, such as snow water equivalent and density, for five winter datasets taken at Col de Porte, France, and at Goose Bay, Newfoundland, Canada. A number of modifications to ISBA's snow model (i.e., new liquid water reservoir in the snowpack, new formulation of snow density, and melting effect of incident rainfall on the snowpack) were found to improve the numerical representation of snow characteristics.

Objective scores for the fully interactive preimplementation tests carried out with the Canadian regional weather forecast model indicated that ISBA's improved snow scheme only had a minor impact on the model's ability to predict atmospheric circulation. The objective scores revealed that only a thin atmospheric layer above snow-covered surfaces was influenced by the change of land surface scheme, and that over these regions the essential behavior of the atmospheric model was not significantly altered by improvements to the treatment of snow cover. It was shown that this lack of response was most likely related to the treatment of the snow cover fraction in each atmospheric model grid tile. The estimation of snow cover fraction relied on simple formulations that were dependent on poorly known parameters, such as the fractional coverage of vegetation. Results showed that uncertainties of only 15% in vegetation fractional coverage could be responsible for uncertainties of as much as 1–1.5 K in screen-level air temperature. This indicates that some care must be exercised in the specification of vegetation and snow cover fractional coverage.

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Clément Chouinard
,
Jocelyn Mailhot
,
Herschel L. Mitchell
,
Andrew Staniforth
, and
Richard Hogue

Abstract

The Canadian regional data assimilation system is described. It is a spinup cycle designed to provide the regional finite-element forecast model with more detailed analyses in a dynamically consistent manner. Its operational performance is evaluated using performance statistics, and a case study is presented to highlight some of the benefits. These include analyses that better fit the data and more detailed and accurate forecasts, particularly for precipitation.

The system also benefits research applications. To illustrate this the authors describe the preparation of the first set of analysts for the international COMPARE (Comparison of Mesoscale Prediction and Research Experiments) Project. The scientific interest of this explosive marine cyclogenetic case is discussed, together with a useful methodology for determining the minimum domain size required by a regional model to avoid forecast contamination from lateral boundaries.

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Sylvie Leroyer
,
Stéphane Bélair
,
Jocelyn Mailhot
, and
Ian B. Strachan

Abstract

The Canadian urban and land surface external modeling system (known as urban GEM-SURF) has been developed to provide surface and near-surface meteorological variables to improve numerical weather prediction and to become a tool for environmental applications. The system is based on the Town Energy Balance model for the built-up covers and on the Interactions between the Surface, Biosphere, and Atmosphere land surface model for the natural covers. It is driven by coarse-resolution forecasts from the 15-km Canadian regional operational model. This new system was tested for a 120-m grid-size computational domain covering the Montreal metropolitan region from 1 May to 30 September 2008. The numerical results were first evaluated against local observations of the surface energy budgets, air temperature, and humidity taken at the Environmental Prediction in Canadian Cities (EPiCC) field experiment tower sites. As compared with the regional deterministic 15-km model, important improvements have been achieved with this system over urban and suburban sites. GEM-SURF’s ability to simulate the Montreal surface urban heat island was also investigated, and the radiative surface temperatures from this system and from two systems operational at the Meteorological Service of Canada were compared, that is, the 15-km regional deterministic model and the so-called limited-area model with 2.5-km grid size. Comparison of urban GEM-SURF outputs with remotely sensed observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) reveals relatively good agreement for urban and natural areas.

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